Improved FileReader class and fixed issue in AttachmentPage

This pull request fixes an issue in ```AttachmentPage``` (```'``` vs. ```"```).

It also improves the ```FileReader``` class. To comply with util classes like ```XML``` and ```XMLWriter``` I
* removed the ```final``` tag of the class
* changed the access level of the attributes and methods from ```private``` to ```protected```
* moved some stuff into own methods which can be overwritten in possible sub classes (```sendHeaders``` and ```sendFile```)